Shh signaling is required for OT expansion. Representative examples of the effects of pharmacological loss and gain of functions experiments with the antagonist Cyclopamine (Cyc) and the agonist Purmorphamine (Pur) applied locally in ovo at two developmental time windows. A-C. Effect on the OT morphogenesis observed at ED4 after a pharmacological treatment performed at ED1.5. Bar: 1 mm. D-F. Effect observed at ED7 after a treatment performed at ED5. Bar: 5 mm. Contours of lateral views of OT corresponding to treated embryos were delineated over the control embryos OT (arrow) in order to facilitate a direct comparison and denote the morphological differences between controls and treated embryos. Noteworthy the effect is not restricted to the OT, the dorsal forebrain (future brain hemispheres) and the optic cup (future retina) also underwent significant morphogenetic changes.